This invention relates to linear accelerators (linacs) and more particularly to accelerators with radio frequency quadrupole (RFQ) resonators for controlling the acceleration, focusing and other characteristics of a beam of high velocity charged particles.
An RFQ linac is a structure which has four-pole symmetry and produces focusing, bunching, and acceleration of charged particle beams by the use of radio frequency fields only. No static internal magnetic or electric fields are required as is the case in a conventional RF linac. The four-pole symmetry of the device produces a strong electric quadrupole field in the vicinity of the axis, which can be used to focus and confine charged particle beams. By modulating the pole piece tips as generally described in "RF Quadrupole Beam Dynamics", by R. H. Stokes, K. R. Crandall, J. E. Stovall, and D. A. Swenson, IEEE Transactions on Nuclear Science, Vol. NS-26, No. 3, June 1979, a longitudinal component of electric field is produced, which can be used to bunch and acelerate the beam. The modulation on the pole piece tips also produces strong alternating gradient focusing of the beam. Thus, the RFQ is capable of focusing, bunching and accelerating charged-particle beams to high energy.
Since the RFQ linac uses electric fields for focusing, it has stronger focusing power for low velocity particles than the conventional magnetic quadrupole, and therefore lower injection energy is required than with a conventional linac. In addition it can be designed to adiabatically bunch the beam producing near-perfect beam capture.
The basic components of an accelerator (as illustrated in FIG. 1) include a source of "low" velocity electrically-charged particles, external focusing magnets, one or more RFQ resonators in which the particles are bunched, focused and accelerated, control magnets and a target or experimental chamber. For the RFQ resonator, the control operations are accomplished by means of a radio frequency electric field. The modulations between adjacent electrodes or rods are spaced a distance apart equal to the distance a particle would travel in half of a radio frequency period. Thus the particle travels to the next set of modulation in correct radio frequency phase to gain energy from the electric field and also, receive alternating-gradient transverse electrical focusing forces. As the particle gains energy, it is also required that the spacing between modulation of the rod be increased so that the particle stays in synchronism with the periodic changing electric field.
RFQ resonators are often designed with a long cylindrical cavity whose axis is the same as the axis of the accelerated beam. In one design, as illustrated in FIGS. 2 and 3, four vanes are equally spaced at the inner wall of the chamber or cavity and extend along the length of the cavity. These vanes are fixed to the inner chamber wall and taper inwardly towards the center of the cavity. The vane tips are modulated in order to form and shape the electric field to provide the desired bunching, focusing and acceleration of the particles. The electric fields between the tips of adjacent vanes are as illustrated in FIG. 3. The radio frequency quadrupole mode, i.e., that which is required for proper operation, is essentially the perturbed TE.sub.210 cylindrical resonant cavity mode excited at the cut-off frequency by proper end termination, essentially an "open circuit".
One of the problems associated with this design relates to the existence of a plurality of resonant frequencies which may occur in addition to the desired frequency of the quadrupole mode. These include resonances such as TE.sub.11N and TM.sub.01N which naturally occur lower or higher in frequency than TE.sub.210 depending on the value of N. However, they are brought near the desired TE.sub.210 resonant frequency because of perturbing effects of the vanes. In addition, higher order TE.sub.21N modes are also possible. In all the above, N is the number of half wavelength variations in the axial direction. With these other resonant frequencies near that of TE.sub.210 ; adjustment, control, stability and operation of the resonator become difficult because the other resonant frequencies interfere with proper excitation of the desired TE.sub.210 mode. Any mechanical changes with time or temperature cause these interfering resonances to change with respect to each other and the desired TE.sub.210 mode.
In another design as illustrated in FIGS. 4 and 5, the modulated electrodes or rod elements are located close to the central axis. In this design, adjacent rods are shorted to different sections of the outer shell and the rod ends make no contact with the cavity axial ends. The shorting elements (67 and 68) form the inductance "L" of the circuit with the spacing between adjacent rods forming the capacitance, "C". The circuit is resonant as a parallel LC circuit. The magnetic field lines are characterized by longitudinal paths concentrated near the inside of the shell parallel to the axis. The magnetic lines loop around at the cavity ends down the other side and around again. This design also has interfering modes much as the previous design close to the desired quadrupole mode when the structure is made longer for higher energy output.
One object of the invention is an RFQ resonator with fewer interfering modes. A second object of the invention is an RFQ resonator for a linear accelerator. A third object of the invention is an RFQ resonator which may be of reasonable size while providing the desired focusing, bunching and accelerating of the particles and especially heavy ions. A further object of the invention is an RFQ resonator which may be extended in length and include multiple resonating units without inducing any interfering mode. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention.